Horse thistle - Onopordum acaulon
Effective: May 13, 2015
Taxonomic Position: Asterales : Asteraceae
Pest Type: Plants
Pest Code (NAPIS): PBFCNDA
These Approved Methods are appropriate for: 2024, 2023, 2022, 2021, 2020, 2019, 2018, 2017, 2016, 2015, 2014, 2013, 2012, 2011
||NAPIS Survey Method
||Collect suspect plants.
||3031 - General Visual Observation
The following are recommendations for executing the survey using the approved methods for pest surveillance. The recommendations are developed through literature review and consultation with subject matter experts.
Found in pastures, roadsides, disturbed areas, irrigated vegetable crops, and cultivated land.
Develops a large rosette that may spread to cover an area up to 60 cm or more in diameter.
The numerous whitish-green leaves are densely covered in woolly hairs. They are deeply divided with numerous spiny waxy lobes.
Flowers are borne in globular flower heads 4 to 6 cm in diameter. Each head contains white or purple florets and is surrounded by sharp spines. Several flower heads are formed as a cluster at the center of the rosette, close to the ground. They flower heads have no stalks.
Seeds are gray or brown in color and about 4 mm long. They are four-sided with a ridged seed coat and 25 mm long pappus (parachute) of pale barbed hairs.
Key Diagnostic or Identification
Morphological: Mature flowering plant is readily identified.
The Federally Noxious Weeds Disseminules of the U.S. provides keys and fact sheets that can help identify seeds of Federally Noxious Weeds, including O. acaulon.
During seedling and rosette stages: Onopordum acanthium, Onopordum illyricum